ENZYME ACTION
Enzymes are folded in GLOBULAR SHAPES. The enzyme’s shape
enables it to receive only one type of molecule; that molecule that
will fit into it’s shape. The place where the substance fits into the
enzyme is called the active site and the substance that fits into the
active site is called the substrate.
Enzyme action occurs when the enzyme and substrate collide.
During the collision the substrate slots into the active site of the
enzyme. Collisions happen because of the rapid random movement
of molecules.
When the substrate joins with the enzyme the entire structure is
called the enzyme-substrate complex. The substrate
becomes changed by the enzyme’s action and is then releases as
the product. The enzyme is then free to join another substrate.
Enzymes can be either anabolic or catabolic. The same enzyme can
be used to form smaller molecules from a larger molecule or to do
the opposite.
An example of a catabolic enzyme is amylase. Amylase converts
starch into maltose.
An example of an anabolic enzyme is DNA polymerase. This
enzyme repairs (rebuilds) DNA.
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TYPES OF ENZYME ACTION
As stated previously, the substrate must fit into the enzyme at the
active site. Some substrates fit nicely into the active site. This
situation is called the Lock and Key Model.
LOCK AND KEY MODEL
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The enzyme is a complex protein molecule, but there is a
particular site where the reactant molecule 'docks in' by random
collision. The enzyme is sometimes referred to as the 'lock' and the
initial reactant substrate molecule as the 'key', hence this is called
the ’lock and key’ mechanism. This is also explains whyenzymes
are very specific (Enzyme Specificity). You need the right
molecular key for a particular molecular lock.
Even when different substrate molecules are present, only those
that have the specific shape complementary to the active site are
able to bind with the enzyme's active site.
Sometimes the shape of the active site must be slightly changed.
This situation is called the Induced Fit Model.
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INDUCED FIT MODEL
The enzyme’s active site has a shape closely complementary to
the substrate The substrate locks into the active site of the
enzyme. The active site alters its shape holding the substrate
more tightly and straining it. An enzyme-substrate complex is
formed. The substrate undergoes a chemical change and a new
substance, product, is formed. The product is released from the
active site. The free unaltered active site is ready to receive a
fresh substrate.
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NAMES OF ENZYMES
Enzymes are named by their substrate. The letters ase are
added to the substrates name. Examples are:
lactase – breaks down lactose (milk sugars)
diastase – digests vegetable starch
sucrase – digests complex sugars and starches
maltase – digests disaccharides to monosaccharides (malt
sugars)
glucoamylase – breaks down starch to glucose
protease – breaks down proteins found in meats, nuts, eggs,
and cheese
lipase – breaks down fats found in most dairy products, nuts,
oils, and meat
cellulase – breaks down cellulose, plant fibre; not found in
humans
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GENERAL EFFECTS OF ENZYMES
1. Enzymes lower the activation energy. This is the energy input
needed to bring about the reaction. Enzymes enable the reaction to
occur with less energy than would be needed if the enzyme were not
present.
2. Regulate the thousands of different metabolic reactions in a
cell and in the organism.
3. The activity of a cell is determined by which enzymes are active in the
cell at that time.
4. Cell activity is altered by removing specific enzymes and/or
synthesising new enzymes.
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INHIBITORS
Enzyme inhibitors are molecules that interact in some way with the
enzyme to prevent it from working in the normal manner. Poisons
and drugs are examples of enzyme inhibitors. Inhibitors change the
shape of the enzyme and make it nonuasable to a substrate.
Inhibitors can also act as a substrate and bind to the enzyme. This
prevents the enzyme from binding with its intended substrate.
When this happens the enzyme is said to be denatured.
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FACTORS THAT AFFECT ENZYME ACTIVITY
TEMPERATURE
At 0°C enzyme action is low because the movement of molecules
is low. This causes the collision frequency between enzyme and
substrate to be low. Increasing the temperature speed up the
movement of molecules and thus the collision frequency
increases therefore enzyme action increases. Human bio enzymes
work best at 37 degrees Celsius. As the temperature raises the
shape of the enzyme changes and the enzyme
becomes denatured. Temperature above 50 degrees Celsius will
denature most human enzymes.
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pH
Most enzymes work best at a pH of 6-8. When the pH is outside
this range the enzyme will lose its shape and
become denatured. The ideal (optimum) pH for most enzymes
is 7.
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Some enzymes work best at other pH levels. The following graphs demonstrate
this:
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ENZYME CONCENTRATION
As the concentration of an enzyme increases the rate of reaction also
increases, provided that the substrate is in excess.
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SUBSTRATE CONCENTRATION
At low concentration of substrate an increase in concentration will
cause an increase in the rate of reaction. However, once the
concentration is such that all the active sites of the enzyme are
constantly in use then further increase in substrate concentration
will have no effect on the rate of reaction.
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IMMOBILISED ENZYMES
Immobilized enzymes are enzymes which may be attached to
each other, to insoluble materials, or enclosed in a membrane or
gel. This can provide increased resistance to changes in
conditions such as pH or temperature. It also allows enzymes to
be held in place throughout the reaction, following which they are
easily separated from the products and may be used again.
Immobilised enzymes are used in bioreactors .These procedures
are used to produce many products which used to use microorganisms. See the bacteria webpage for a discussion of
bioreactors.
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TYPES OF IMMOBILISATION
Adsorption: In this method the enzyme is attached to a
support. Supports can be ceramics, glass, or plastics.
Membrane Enclosure: In this method the enzyme is
enclosed in a porous membrane.
Gel entrapment: The enzymes are held in a gel. Sodium
alginate is a common gel used. The gel allows the
substrate to enter and the product to leave.
Chemically bonded to a support or to each other: See
textbook page 95.
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ADVANTAGES OF IMMOBILISATION
1. It makes for easier purification of the product as the separation of the
enzymes from the products is easily accomplished.
2. It is easy to recover and recycle the enzymes. This leads to a more
economical process.
3. The enzymes remain functional for much longer as it is a gentler
process.
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USES OF IMMOBILISED ENZYMES
The following products are derived from immobilised enzyme
action:
1. Fructose derived from glucose: Fructose is sweeter than glucose and
is used in soft drinks and other sweet products.
2. Antibiotics: Enzymes are used to change penicillin into new, wider
used, antibiotics.
3. Sewage Treatment: Instead of bacteria enzymes can be immobilised
and used.